Electronic and communications systems generally include a chassis having a plurality of slots for receiving electronic cards, for example, printed wiring boards (PWB), also known as printed circuit boards (PCB). The cards are inserted into the slots, substantially parallel to one another. The slots are manufactured by high precision machinery and have very precise dimensions, e.g., width of tolerance .+-.0.05 mm. While it is possible to manufacture electronic cards having very precise thickness, it is very expensive. Hence, the thickness of the cards to be inserted into the slots generally varies greatly. For example, for a nominal thickness of 1.60 mm, the thickness of the cards can range from 1.36 mm to 1.86 mm. The result is that the cards inserted into the slots are loosely retained in the track and not precisely positioned, which is problematic in many electronic systems. In addition, the cards themselves are flexible, and can be twisted as much as 1% during the manufacturing. This problem is particularly serious in high frequency telecommunications systems, where inaccurate seating between adjacent cards can cause random air gaps and consequent radio frequency interference (RFI) problems.
There are known in the art a number of spacer elements which serve to hold adjacent cards affixed to one another at a predetermined distance, such as those shown in U.S. Pat. Nos. 3,983,459 and 4,444,318, and 4,604,776. These maintain a fixed minimum distance between adjacent cards, but are not designed to provide, and do not ensure, consistent alignment.
There are also known retention elements for retaining the cards in their slots, such as a ball with a spring, or a leaf spring mounted on the bottom and/or top of the card. These devices provide controlled spacing by pressing against the wall of the slot. However, these springs are not fixed, so they tend to wear rapidly and cease functioning under frequent insertion and removal of the card. They also lack the necessary strength to maintain a card in a fixed position, when opposed by RFI gaskets. There are also known wedges for inserting between the slot and the card to provide thermal transfer and to control vibration. But these wedges are expensive to manufacture.
Accordingly, there is a long felt need for a simple, inexpensive device to ensure precise placement of electronic cards in their slots.